Is there a way to abort the 'Payback' story mission?

I started the story mission 'Payback' in Far Cry 4, however I would like to abort it because:

I believe it will remove Yuma and weaken her fortress, and I would like to liberate the fortress at full strength

I have canceled missions before from the Escape menu, but this time my only options are: "Resume" and "Restart Mission":

I have tried:

• fast traveling
• starting a side quest (eg Kyrati racing)
• dying
• leaving the area
• exiting to menu
• exiting the game
• starting the game in online mode with the hope of inviting someone (anyone!) to the game, thereby disabling the campaign temporarily

Another user had this issue, but someone else reported that "Leave mission" works for them.

Is there something I have to do before I can leave the mission? I'm open to just about anything up to and including editing the save file*, if needed.

^{* Unfortunately I don't have a backup of the save, because it seems that Uplay keeps saves in its directory instead of %USERPROFILE% (harrumph).}